Computer systems typically include a central processing unit (CPU), devices within the computer (such as memory storage devices), and one or more interfaces with peripheral devices optionally attached to the computer. Computer systems use buses to carry the signals between these various components of the computer system allowing the CPU and the devices to communicate with each other. Recent advancements in the computer industry have resulted in faster buses. One of these faster buses is the Peripheral Component Interconnect (PCI) bus that has developed into an industry standard bus.
The recent development of the PCI bus as an industry standard has created the problem of how to block access to a computer device coupled to the PCI bus. Some access operations over a PCI bus may require blocking access to one or more of the other devices connected to the PCI bus, while allowing a requested device to receive the access. For example, in a computer system with a PCI bus, and two sets of DMA controller devices connected to the PCI bus, an operation might require accessing a first DMA controller on the ISA bus without accessing the second DMA controller (i.e., blocking access to the non-required DMA controller devices coupled to the PCI bus).
Blocking access to a device on a PCI bus presents difficulties due to the nature of how devices interface over a PCI bus. The address phase on a PCI bus appears on the first clock. Therefore, the method of blocking access by blocking an address during a clock other than the first is not viable over a PCI bus. Furthermore, because the first clock on a PCI bus happens relatively quickly, it is difficult to block the address during the first clock in order to block access to the device. A solution to this problem is to slow down the bus by one clock and then block access during the second clock. However, this presents two problems because to do this you must 1) slow down bus (which defeats the purpose of having a faster PCI bus) and 2) use a great deal of hardware to implement this solution. A technique for blocking access to a device without slowing down the PCI bus with minimal hardware support would provide an advantage when using a computer system having a PCI bus.